Prolactin is a hormone that is produced in the anterior lobe of the pituitary gland and derives its name from its role in the initiation of lactation. Other possible regulatory functions of prolactin are not entirely clear. Prolactin is produced in both males and females and is present in elevated levels in pregnant and lactating females and in patients with prolactin-secreting pituitary tumors.
Human prolactin (hPRL) is a hormone comprising a single protein chain with a molecular weight of about 23,000. Its structure has been elucidated and published, Cook et al., J. Biol. Chem., 256:4007-4016 (1981).
A radioimmunoassay, which has become a standard assay for determining hPRL levels in biological samples, was described in Sinha, et al., J. Of Clin. Endocrinology & Metabolism, Vol. 36, No. 3, March 1973, pp. 509-516. This immunoassay uses hPRL obtained from pituitary glands of recently deceased persons as standards for assays, as labeled peptide in the radioimmunoassay and to induce production of antibody to hPRL in host animals. Not only is obtaining hPRL from its natural source a continuous problem, but human prolactin must be isolated and substantially purified by relatively tedious procedures. The activity of the isolated prolactin must then be determined on a lot-by-lot basis.
Furthermore, when hPRL is obtained from human pituitary glands, it is invariably contaminated with other pituitary substances, including other hormones. This poses problems in the production of antisera because antisera induced by even a slightly impure prolactin fraction contain antibodies to the contaminating pituitary substances. With careful purification, such contamination can be minimized; for example, the antiserum described in Sinha et. al. supra. publication had less than two percent cross-reactivity with human growth hormone. However, high purity hPRL is isolated only with the exercise of great care but with low yield, and the cross-reactivity of antisera for interfering substances must be determined on a lot-by-lot basis.
Thus while the previously described assay for hPRL is sensitive and has gained widespread acceptance, its accuracy is subject to the careful performance of preparatory and quantitative procedures. It would be desirable to have a radioimmunoassay for hPRL which does not require human pituitary extracts and which is less dependent on careful prior isolation and prior quantitation. Further, highly purified hPRL is difficult to iodinate, and radioiodinated hPRL tends to be unstable.